This invention relates to hydraulic systems for vehicles having reel mower cutting units.
Reel mowers are typically used to mow grass in well groomed areas such as golf courses. Some vehicles which are adapted for operating reel mowers include a hydraulic system which powers the reel mowers and also the vehicle's drive wheels. These hydraulic systems typically include a fluid reservoir which stores and recirculates hydraulic fluid. The reservoir can generally cool the fluid stored therein and thereby maintain fluid in the system at sufficiently low temperatures during operation.
One type of conventional reel mower vehicle includes reel mower cutting units located at the front of the vehicle, which allows the cutting units to mow grass before the vehicle wheels trample or compress the grass. A hydraulic gear pump is positioned near the front portion of the vehicle for powering the cutting units. A charge pump, hydraulic pump and hydrostatic system are also carried near the front portion of the vehicle for powering the vehicle's ground engaging drive wheels. Fluid within the hydraulic system becomes heated under operational loads. Also, many of the hydraulic components and lines are positioned within or relatively close to the warm engine compartment located near the front portion of the vehicle, which further heats the hydraulic fluid flowing in the system. Once the hydraulic fluid has passed through the pumps and other hydraulic componentry it is routed back to the reservoir via a common return line. Conventional hydraulic systems typically position the reservoir near the front portion of the vehicle, and relatively close to or within the engine compartment in order to make the vehicle relatively compact. The reservoir's close proximity to the warm engine can generally hinder the fluid within the reservoir from cooling. A hydraulic fluid cooler may therefore be necessary, adding further expense to the vehicle.
The gear pump which operates the reel mowers, and the hydrostatic pump which operates the drive wheels have different operating flow rates or capacities, and operate different vehicle components. The two pumps and their respective motors therefore generally act as separate systems. Many conventional systems connect the two pumps in parallel such that the two pumps draw fluid from the reservoir via a common input line. The parallel configuration of the two pumps allows each pump to draw the proper amount of fluid it requires from the reservoir via the common input line even though the pumps draw in fluid at different rates. Since the input line supplies both pumps with fluid, a relatively large amount of fluid must be drawn in from the reservoir through the input line.
Cavitation is a phenomenon which can occur when the pressure head in a hydraulic system is insufficient and the fluid is allowed to reach its vapor pressure. The combination of a long suction line and a pump with a marginal inlet vacuum is an ideal environment for cavitation to occur. When the hydraulic fluid is relatively cold, the fluid viscocity increases, making it more difficult for a pump to draw in fluid. Cavitation causes fluid to vaporize into vapor cavities which are carried with the flowing liquid. As cavities encounter a region of high pressure, they may collapse as the vapor condenses. This collapse is accompanied by intense pressures caused by fluid rushing in to fill the cavity where its momentum is converted into pressure from the impact of the walls of the cavity as they meet. Any boundry or wall containing the fluid in the vacinity of the collapse will be subjected to repeated and intense localized pressures, which may cause damage to components, pumps or fluid lines. Conventional reel mower hydraulic systems generally do not experience many pump cavitation problems. Most conventional reel mower hydraulic systems are relatively compact, and therefore do not include long hydraulic lines which would significantly contribute to problems of cavitation.
It would be desirable to provide a hydraulic system which operates a plurality of components such as reel mower cutting units and vehicle drive wheels, and which generally cools the hydraulic fluid in a reservoir while hindering or preventing the occurance of pump cavitation, even when operating at relatively low ambient temperatures.